Operation response slowly

After loading the file .dspx which includes some shp layer, I add one drawinglayer to the mapcontrol, and make it add some features then remove them among certain interval in the timer event, Now I try to pan the map or zoomin or zoomout , it response
very slowly. I don't know why, if just loaded the file .dspx, the operation of pan or zoomin or zoomout is working very smoothly!